OBSERVED CHANGES IN THE DIURNAL TEMPERATURE AND DEWPOINT CYCLES ACROSS THE UNITED-STATES

We analyzed hourly coterminous U.S. airport data, beginning in 1948, f or changes in daily temperature and dewpoint regimes. We found an ubiq uitous phase shift in the daytime cycle of warming, towards later in t he day. Overall day and night temperature changes were very consistent with the results of Karl et al. [1993] even though this is an entirel y different source of data, and one that is not specifically deurbaniz ed, although we attempted to use stations that were not from downtown airports. We divided our results into eastern and western subsets as a first approximation for high and low sulfate aerosol conditions, and found evidence that was consistent with a sulfate effect on overall te mperature, but inconsistent with modeled estimates of the effect of su lfates on the intradiurnal regime. It is difficult to establish the ca uses of the observed intradiurnal phase shift of warming time although urbanization and anthropogenic emissions are likely to be involved. T he nature of our finding is subtle; and is consistent with other resul ts that show very modest (and nonobvious) responses to greenhouse chan ges. Further research will ultimately clarify its causes and effects.